1. Field of the Invention
The present invention relates to a method for producing carbon fiber, more specifically to a method for stably producing carbon fiber from various types of pitches, without causing breakage of fiber and in mass quantities at a fast speed. The present invention also relates to a method for producing carbon fiber suitable for the production of prepregs and superior in spreading ability of filaments.
2. Description of the Related Art
It is well known that carbon fiber is widely used as a prepreg material. Carbon fiber suitable for prepregs is required to be free from fluff in the fiber bundles, superior in the spreading ability, and long in length.
Pitch fiber, however, is fragile. Even the intermediate process product of infusible fiber is remarkably fragile with a strength of 5 to 10 kgf/mm.sup.2, so an infusible fiber yarn comprised of 100 to 100,000 of the filaments bundled together is difficult to handle, easily breaks, a long fiber is difficult to make and there is a lot of fluff.
Further, if the infusible fiber yarn is linearly fed out for carbonization or graphitization, then, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-91229, during the carbonization or graphization, problems will arise that the individual filaments in the fiber yarn will fuse together, the yarn will become rigid, and the quality of the product will otherwise remarkably fall.
In regard to these problems, Japanese Examined Patent Publication (Kokoku) No. 62-20281 describes a method of initial carbonization in a range of 400.degree. to 650.degree. C. after the infusibilization and then handling this initially carbonized fiber. In general, if pitch fiber is treated not to fuse and then carbonized, there are points where the elongation at break of the filaments remarkably rises in a range of temperature of the carbonization treatment of 500.degree. to 600.degree. C. It is known to use this large elongation at break, perform the carbonization in this temperature range, and handle the yarn at that time.
Further, Japanese Unexamined Patent Publication (Kokai) No. 60-126324 describes a method of heating until the elongation at break of the fiber exceeds 2.5%, performing the carbonization, then further performing carbonization and graphitization of the fiber yarn under tension at a higher temperature to produce a carbon fiber bundle (or multifilaments) with a good filament alignment.
On the other hand, as a means for shortening the infusibilization time and otherwise improving the productivity or improving the physical properties of the carbon fiber, a method of using nitrogen dioxide gas for the atmospheric gas at the time of infusibilization is described in Japanese Unexamined Patent Publication (Kokai) No. 60-259629, or a method of further mixing steam in the nitrogen dioxide is described in Japanese Unexamined Patent Publication (Kokai) No. 2-6618.
By using a gas comprised of a mixture of nitrogen dioxide gas in the atmospheric gas at the time of the infusibilization, it becomes possible to remarkably shorten the infusibilization time compared with the infusibilization using air as an atmospheric gas, which has been the general practice in the past.
When using nitrogen dioxide for the atmospheric gas at the time of infusibilization, even if the initial carbonization is performed in the range of 400.degree. to 650.degree. C. as has been generally known in the past, while the elongation at break of the fiber is improved, there are problems that, in this range, the breaking strength is low, yarn breakage occurs, and the handling of the yarn is not improved.
Further, due to the shrinkage of the fibers, the alignment of the fibers was poor and the physical properties appeared unstable. Due to the breakage of the yarn in the graphization process, a reduction in the productivity was caused.
As a method for obtaining a long fiber from fragile pitch fiber, further, Japanese Examined Patent Publication (Kokoku) No. 51-12740 discloses a method of depositing spun fiber on a wire box, performing a infusibilization on the fiber in the state deposited on the box, performing carbonization at a temperature of at least 700.degree. C. to increase the tensile strength of the fiber, then performing carbonization at a temperature of 1500.degree. C. in the linear form. With this method, however, flaws were formed in the yarn at the time of deposition at the stage of the initial carbonization. These flaws were not easily eliminated even in the later carbonization. Therefore, it was only possible to obtain a fiber inferior in terms of the yarn fluff and spreading ability. Japanese Unexamined Patent Publication (Kokai) No. 64-33214 discloses a method of performing carbonization during the shrinkage of the fiber after the infusibilization, that is, at not more than a temperature of 1300.degree. to 1500.degree. C., under a tension of 0 to 50 mg/denier, then performing the graphitization at a degree of stretching 0.1 to 1.2% higher than the spontaneous degree of stretching. With this method, however, when performing the infusibilization by a gas containing nitrogen dioxide, it was not possible to stably obtain a fiber superior in spreading ability.